Setting aside the fact that gravity is not part of this theory,
the Standard Model on its own leaves many questions unanswered.
For example, why is the top quark so heavy? It is nearly 40 times
more massive than the next heaviest, the bottom, and over 40000
times more massive than the light up and down quarks.
Another problem is that fundamental spinless bosons are kept in
a stable theory only through very tenous fine tuning of the
theory's parameters. Could the Higgs be a composite particle?
Is there a more suitable mechanism for electroweak symmetry breaking?
Or can some extra supersymmetry stablize the theory?

It should be emphasized that the Standard Model describes all of
the observed interactions of elementary particles to date (Sep. 2000).
Some of these experiments are the most precise experiments ever performed.
Therefore, it is highly unlikely that it will be disproved.
The present searches for new physics beyond the Standard Model
all have strong theoretical motivation, but are not inspired by
a conflict between experimental results and theoretical calculations.
So any new theory of fundamental particles and their interactions
must necessarily subsume the Standard Model.